Patients from kindreds with MEN 1, MEN 2A, and MEN 2B will be screened for tumors by routine biochemical and provocative tests. As a part of the study patients previously having total thyroidectomy for medullary thyroid carcinoma who have developed elevated serum calcitonin levels after surgery will undergo repeat neck exploration and aggressive resection of lymph node tissue. All patients will have blood drawn for genetic studies, in which attempts will be made to identify the specific genetic abnormalities associated with each of the syndromes. Analysis of constitutional and tumor tissue DNA should allow us to determine the molecular basis of each of the MEN syndromes, and to develop a method for preclinical and even prenatal detection of affected individuals.